Cathode catalyst layers modified with Brønsted acid oxides to improve proton exchange membrane electrolysers for impure water splitting

Proton exchange membrane (PEM) electrolysers typically use ultrapure water as feedstock because trace contaminants in feedwater, especially cationic impurities, can cause their failure. Developing PEM electrolysers that can withstand lower-purity water could minimize water pretreatment, lower maintenance costs and extend system lifetime. In this context, we have developed a microenvironment pH-regulated PEM electrolyser that can operate steadily in impure (‘tap’) water for more than 3,000 h at a current density of 1.0 A cm−2, maintaining a performance that is comparable to state-of-the-art PEM electrolysers that use pure water. Using a technique that combines a pH ultramicroelectrode with scanning electrochemical microscopy, we monitored the local pH conditions in a PEM electrolyser in situ, finding that Brønsted acid oxides can lower the local pH. We thus introduced a Brønsted acid oxide, MoO3−x, onto a Pt/C cathode to create a strongly acidic microenvironment that boosts the kinetics of hydrogen production, inhibits deposition/precipitation on the cathode and suppresses the degradation of the membrane.